In 1997 Stewart took three weeks' leave from the public relations company Hill & Knowlton to help his friend Martin Bell who was standing for Parliament in Tatton as an Independent candidate. Stewart was alongside Bell when they were confronted by the sitting Conservative Member of Parliament for the constituency, Neil Hamilton and his wife Christine on Knutsford Heath.[22] Bell, who was opposing Hamilton as a result of accusations that Hamilton had accepted money for promoting causes in Parliament, gave Stewart the credit for defining his criticism of Hamilton as having already admitted to "conduct unbecoming".[23]

Since leaving the army Stewart has become a well-known commentator upon military and political affairs, frequently commenting upon the defence policy of the British Government and the wars in Afghanistan and Iraq.[24] In 2009 he condemned the system compensating injured soldiers, accusing the MOD of acting with "the speed of a striking sloth".[2][25][26] He also accused the government of repeatedly refusing the requests of army commanders for more troops and more helicopters in Afghanistan.[27]

On 28 July 2009, it was revealed that he had been approved to put himself forward for selection to constituency associations as a PPC for the Conservative Party. As the former commander of the Cheshire Regiment he was linked to the safe Conservative East Cheshire seats of Macclesfield[28] and Congleton,[29] however the final shortlists for Macclesfield and for Congleton from Conservative Central Office did not contain his name. In summer 2009 he was shortlisted for Beckenham, one of the safest Conservative seats in the country,[30] and on 6 December it was announced that he had been selected as Conservative candidate there, winning an overall majority on the second ballot.[31]

At the 2010 general election on 6 May, Stewart was elected as the new MP for Beckenham.[32][33]

Stewart served on the House of Commons Defence Select Committee and in May 2014 he was one of seven unsuccessful candidates for the chairmanship of the committee.[34]

Stewart has been vocal in criticising cuts to the defence budget, suggesting in March 2015, that if the Joint Chiefs of Staff were to resign over the issue it would "make a very powerful message". When asked if he would resign as an MP himself, he said that he was considering it.[35]

In 2016, Stewart was criticised after it was revealed he had described Isabel Hardman, The Spectator's assistant editor, as "totty". He was accused of being sexist and for making Parliament an uncomfortable place for women to work. He later apologised to the journalist, but went on to publicly defend himself against an environment that he described as overly 'politically correct'.[40][41]

In 2017, Stewart's name was included on a leaked internal list of Conservative Party MPs who has been alleged to have acted inappropriately[42]. He publicly denied accusations that he was regularly inebriated in Parliament and was inappropriate with female colleagues[43]. Unlike some of the other MPs featured on the leaked list, he was not suspended or subject to further documented disciplinary action.[44]

Stewart employs his wife as a Senior Parliamentary Assistant on a salary up to £30,000.[45] He was listed in an article in The Daily Telegraph criticising the practice of MPs employing family members, on the lines that it promotes nepotism.[46] Although MPs who were first elected in 2017 have been banned from employing family members, the restriction is not retrospective - meaning that Stewart's employment of his wife is lawful.[47]

He lives in Beckenham in London. He has six children and is married to Claire Podbielski, whom Stewart met whilst he was commanding British forces in Bosnia in 1993. He was married to Elizabeth Hoffman when he started the relationship with Ms Podbielski and later described the tabloid coverage of his affair as:"bloody awful for my family, friends and for me."[48] Podbielski is Polish-Swiss and now works as Stewart's Senior Parliamentary Assistant; she is a former Delegate of the Red Cross.[45][48]